Generally, as for an audio system including a plurality of audio sources such as a radio, cassette player and CD player, a user must carry out volume control every time he or she selects the audio source to adjust the volume of a speaker at a desired level. In other words, since the volume level varies in accordance with the audio source, the user must perform the volume control every time the audio source is selected, and the volume control is usually troublesome.
To eliminate such trouble as the volume control, there is an apparatus, for example, which compares a set volume level stored in a volume level memory circuit with a volume level of an audio signal detected by a volume level detecting circuit, and controls a volume control circuit in response to the difference. In this case, a user stores the set volume level into the volume level memory circuit by setting the volume level of a receiving apparatus such as a television and radio or of an audio system such as a CD player with a volume controller. When the volume level of the audio signal detected by the volume level detecting circuit differs from the set volume level, the volume control circuit is controlled to obtain the sounds of the set volume level (see Patent Document 1, for example).
Furthermore, to obtain the appropriate speaker output without the user's resetting of the volume when the audio source is changed, there is another apparatus which detects the signal level of the input signal with a signal level detecting means, and stores the signal level into a memory means, and which compares the signal level detected by the signal level detecting means with the signal level read from the memory means, and controls the gain of a variable gain means in response to the compared results in such a manner that the speaker output is nearly equalized before and after the change of the audio source (see Patent Document 2, for example).    Patent Document 1: Japanese patent application laid-open No. 6-37563/1994 (pp. 3-5 and FIG. 1-FIG. 7)    Patent Document 2: Japanese patent application laid-open No. 2003-348682 (pp. 3-4 and FIG. 1-FIG. 2)
With the foregoing configurations, the conventional audio systems must store the set volume level in advance, and control the volume control circuit by successively comparing the volume level detected by the volume level detecting circuit with the set volume level. Therefore the conventional audio systems have a problem of not only increasing the load in the control system at the volume control, but also of being very troublesome when the number of the audio sources is large because the user must set their volume levels in advance.
In addition, the conventional audio systems compare the signal level detected by the signal level detecting means with the signal level read from the memory means, and control the gain of the variable gain means in such a manner as to nearly equalize the speaker output before and after the change of the audio source. However, the conventional audio systems have a problem of increasing the load of the control system at the volume control because they must successively compare the signal level detected by the signal level detecting means with the signal level read from the memory means.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide an audio system that has a plurality of audio sources, and enables a user to carry out the volume control much easier when playing the audio selectively from the plurality of audio sources.